SCIENCE · INVESTIGATE AND EXPLAIN

Test an electron budget before assigning geometry

Distinguish bonding electrons, directional domains and a feasible central allocation.

Learning outcome: Compare CO2, NH3, H2O and NO2 at neutral charge. Retain electron budgets and identify lone pairs versus an odd electron. Apply charge+1 to the fixed CO2 skeleton and explain the rejected negative remainder.

Starting recap

Review the electron and formal-charge budget, bond multiplicity, signed interaction energies and the distinction between a model and a measured property.

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Try

Neutral CO2 has16 valence electrons,8 in two double bonds and8 in oxygen lone pairs; its central atom has two bonding domains, not four.

Open the experiment

Predict

Does a double bond count as two separate directional domains around its central atom?

Your prediction and reason

Experiment

Compare CO2, NH3, H2O and NO2 at neutral charge. Retain electron budgets and identify lone pairs versus an odd electron. Apply charge+1 to the fixed CO2 skeleton and explain the rejected negative remainder.

Test a fixed Lewis electron skeleton

Worked teaching model · synthetic values · References CO2, NH3, H2O, BF3, SF6, NO2 with fixed bond orders and terminal lone pairs. Changing net charge only tests the remaining electron ledger, not a stable charged-species prediction. Negative remaining count is explicitly infeasible, never clipped.

Reference skeleton: 0 choice Net charge relative to neutral: 0 e

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Calculated evidence under the declared representation
Quantity	Value	Unit
Total valence electron budget	16	electrons
Bonding electrons	8	electrons
Terminal nonbonding electrons	8	electrons
Algebraic remaining central electrons	0	electrons
Nonnegative allocation feasible	1	flag
Electrons around center if feasible	8	electrons
Bonding domains	2	domains

CO2, two C=O bonds

A multiple bond is one directional bonding domain, although it contains more than one pair. An odd remaining electron is not a lone pair. BF3 and SF6 show that an octet is not a universal law; the ledger does not establish which charged skeleton is stable.

CO2, two C=O bonds

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Begin with conserved electrons

A geometry prediction needs a defensible electron structure. Add neutral-atom valence electrons and adjust for the net charge: add electrons for a negative charge and subtract for a positive one. Then account for bonding electrons and nonbonding electrons without spending the same pair twice. A pleasing drawing cannot repair an inconsistent total.

Compare electrons and domains

Neutral CO2 has16 valence electrons. Two C=O double bonds contain8 electrons, and oxygen lone pairs contain the remaining8. The central carbon has two directional bonding domains. A multiple bond occupies one direction in the elementary VSEPR count even though it contains more than one electron pair. NH3 instead has three bonding domains and one central lone pair; H2O has two bonding domains and two lone pairs.

Test a fixed skeleton rather than assume it

The workbench deliberately holds bond orders and terminal lone pairs fixed while changing the total electron budget. For the stated CO2 skeleton with net charge+1,15 electrons are available but16 have already been allocated. The remaining central count is−1, an infeasible proposal. This is not a negative physical population or proof that a different CO2 ion structure cannot exist. It says that the unchanged drawing cannot represent that electron budget.

Recognize exceptions

The supplied BF3 drawing has only six electrons around boron; the SF6 drawing has twelve around sulfur. The NO2 reference has an odd remaining central electron, which is not a paired lone pair. These examples prevent the octet guideline from becoming an unconditional rule. Expanded-valence Lewis drawings do not by themselves establish extensive d-orbital bonding; that requires a more careful electronic model.

Transfer the method

A new drawing of a neutral species has20 total valence electrons,12 in bonds and6 in terminal lone pairs. The remaining central allocation is2 electrons, one lone pair if that orbital allocation is appropriate. To infer domains, also count bonded directions:12 bonding electrons could arise from different combinations of single and multiple bonds. Electron total alone does not uniquely determine molecular geometry.

Sources and evidence boundary

Conceptual sources checked 30 October 2026: Lewis structures, Geometry. All prose, cases, code and diagrams are original. The supplied models are not measured structures, a performed laboratory investigation or certification of independent mastery. No chemical handling, heating, pressure vessel or cryogenic experiment is requested. Retain predictions, contrasting evidence and a changed-case explanation.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Why does changing total charge not authorize keeping every bond and lone pair unchanged?

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Infer a partial-charge model from bond dipole evidence Next: Rotate a domain model without changing its molecular shape

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